1. Field of the Invention
The present invention relates generally to roadway construction equipment, and more particularly is a multi-dimensional asphalt delivery and compaction system that delivers asphalt to a roadway based on a topographical scan of the road bed.
2. Description of the Prior Art
Various types of equipment are used to provide hard surfaces for streets, highways, parking lots, etc. Included among the broad array of available equipment is an asphalt paver which uses a screed to level a layer, or mat, of asphalt material on an underlying subgrade. Ideally, asphalt paving produces a relatively flat surface in order to provide a smooth ride for vehicles to pass over. Thus, other than for following the gradual curvature of the underlying terrain and for intentional xe2x80x9ccrowningxe2x80x9d, (to encouraging drainage of surface water), the mat placed by the asphalt paver offers an essentially planar surface. This result is optimal if the underlying subgrade has a corresponding planar surface.
After the mat is placed by the paver, the mat is compacted with a heavy roller, which compresses the asphalt maternal to a factor of the thickness of the mat as laid by the paver. If the asphalt material has a uniform density and thickness, which is greater than a certain minimum thickness relative to the size of the aggregate contained in the asphalt material, then the actual thickness of the asphalt mat after compaction depends on the thickness of the asphalt material prior to compaction by the roller. The ratio between (a) the difference in thickness of the mat before and after compaction with the roller, and (b) the thickness of the asphalt mat as placed, is commonly referred to as the xe2x80x9ccompaction factorxe2x80x9d.
If the underlying subgrade and the asphalt material mat are both planar, and if the asphalt material has a uniform density, then the rolled surface will also be planar, as desired. In an actual situation, however, the surface of the underlying subgrade generally has depressions and elevations that cause the surface of the compacted mat to vary substantially from a planar profile. Thus, the asphalt material mat, even though having a substantially planar surface as laid by the asphalt paver, is thicker in some places than in others. As a result, the asphalt, after compaction, no longer exhibits the substantially planar surface but, instead, has depressions and elevations similar to, but less pronounced than, those of the subgrade surface. This uneven result is sometimes referred to as xe2x80x9cdifferential compactionxe2x80x9d.
For example, assume that the desired thickness of asphalt material nominally laid by a paver prior to compaction is six inches. Assume also that the subgrade has a local depression that is two inches deep and a ridge or local elevation that is two inches high. Thus, the thickness of the asphalt material laid by the paver would be eight inches deep over the local depression and only four inches deep over the local elevation. Assume further that the roller compacts the asphalt material to seventy-five percent of its original thickness as laid by the paver, or a reduction in thickness of twenty-five percent. After compaction by the roller, the thickness of the asphalt material over the substantially planar surface of the subgrade would be four and one-half inches.
Similarly, the thickness of the compacted asphalt material over the depression and the localized elevation would be six inches and three inches, respectively. In other words, the surface of the asphalt mat that was substantially planar, as provided by the paver prior to compaction by a roller, now has a surface over the depression that lies one-half inch below the surface of the nominal mat. Further, the surface of the compacted asphalt mat over the local elevation lies one-half inch above the surface of the compacted nominal mat and one-inch above the surface of the compacted mat above the depression. Such a situation obviously does not provide a smooth ride for a vehicle passing over the surface. Ideally less material should be placed over the localized elevation and more asphalt material should be placed over the depression in order to overcome this effect.
The underlying problem with current art pavers is their inability to compensate accurately and adequately to changes in elevation of the subgrade surface. To a large degree this problem is compounded by the fact that modern screeds are only capable of delivering an asphalt mat that exhibits a planar top surface. This method of delivering asphalt is incapable of providing adequate material to overcome the effects of xe2x80x9cdifferential compactionxe2x80x9d. Modern screeds do allow for a certain amount of adjustment vertically, which can be manipulated to provide for a degree of slope and grade along the length and width of the asphalt mat being laid. This however, does not provide adequately for localized variations in the subsurface, such as elevations and depressions in the subgrade. Current art pavers generally use an auger working in conjunction with the screed to provide more or less material to a localized area to compensate for the differences in elevation. This does not provide the degree of compensation necessary to provide a completely smooth driving surface once the asphalt mat is compacted.
Modern pavers can only control the delivery of asphalt along three planer surfaces producing an asphalt mat shaped to the subgrade surface and exhibiting a smooth planar surface. Once this mat is compacted further by a heavy roller it will once again resemble the subgrade only to a lesser degree. What is needed is a method of paving that includes the following steps: 1. Obtaining a topographical profile of the surface to be paved. 2. Processing this information to establish the profile of the surface as it is and the profile of the desired finished surface. 3. Computing the distance between these two surfaces to establish the amount of asphalt with a known compaction factor that will be needed to result in the desired finished surface. 4. Using this information and factoring in the displacement of asphalt material that will take place during the compaction phase to design the profile of the asphalt mat as it should be supplied. 5. A means of manipulating the asphalt mat according to this profile in order to supply exactly the right amount of asphalt material to the subsurface location where it is needed. In reality the mat of asphalt provided for compaction should not be planar as current art pavers provide. Instead it should inversely mimic the characteristics of the subgrade surface to a degree that the shaped mat, once compacted, will attain the smooth surface that is desired.
Accordingly, it is an object of the present invention to provide an asphalt delivery system that supplies an asphalt mat with a thickness that varies according to the subgrade surface variations, thus using xe2x80x9cdifferential compactionxe2x80x9d to build a better road.
It is a further object of the present invention to provide a method of supplying an asphalt mat that provides for a superior planar upper surface following compaction.
It is a still further object of the present invention to provide an asphalt delivery mechanism that includes a means to obtain and store a topographical profile of the subgrade to be covered.
The present invention is a method of obtaining a topographical profile of a road bed, processing that data to generate a road profile for the desired road surface, and then delivering an asphalt mat that varies in thickness according to that profile. The asphalt delivery system enables variance in the mat thickness across the width of the mat as well as in the normal longitudinal direction.
The process is begun by obtaining a three-dimensional profile of the surface to be paved. A scanning means is moved over the road surface to obtain a profile of the entire length and width of the surface to be paved. The scanning means can utilize any of several known means of obtaining a detailed topographical profile, and most often will be radar, sonar, or laser measuring equipment used in conjunction with the Global Positioning System (GPS). The profile data obtained is processed for use in the second phase of the operation.
Data for the profile will be gathered in a manner that will provide data such as elevation, slope, and grade with a resolution scale small enough to produce an accurate representation of the surface to be paved. This data will be used to design a road profile that will control all actions of the paving machine. By figuring the difference between the road profile as it is and the road profile as it is desired to be, and factoring in the correct xe2x80x9ccompaction factorxe2x80x9d we can generate a finished mat profile that will produce the desired road surface. This finished mat profile will utilize the effects of xe2x80x9cdifferential compactionxe2x80x9d in a constructive way and deliver more asphalt material to where it is needed and less to where it is not. This profile will be loaded into the onboard computers of the paving machine and will accurately control the movement of the paving machine as well as the operation of the asphalt delivery mechanism.
In the second phase of the operation, the scanning means is utilized in combination with an asphalt delivery mechanism. The scanning means tracks the exact position of the asphalt delivery mechanism, correlates that to the scanned profile, and thereby controls the operation of the asphalt delivery mechanism. The asphalt delivery mechanism delivers a mat of asphalt of a varying thickness determined by the topographical profile in conjunction with a compression factor for the asphalt material. The thickness is varied not only along the length of the mat, but also across the width of the mat.
The first key component of the variable asphalt delivery mechanism is the inner chamber. This is where an overly thick asphalt mat of a consistent density is formed and made available to the second key component, the variable screed. The variable screed includes a plurality of individual plates that together form a screed the width of the asphalt mat. The individual plates are each attached to a double-action single piston end hydraulic cylinder that moves the plates up and down along an axis perpendicular to the width of the main blade of the asphalt delivery machine. As the asphalt mat is introduced to the variable screed the manipulation of groups of individual plates causes the asphalt material to be removed from the preformed mat in amounts determined by the stored mat profile, thus controlling the profile of the asphalt material output by the system.
An advantage of the present invention is that it makes allowances for variations along the width of the roadbed as well as variations along the length.
Another advantage of the present invention is that the variable screed allows different amounts of asphalt to be deposited along the width of the roadbed.
A still further advantage of the present invention is that the resultant mat is very smooth following compaction.
These and other objects and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.